<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* form表单里面有无输入框聚焦（后代获得焦点），有的话给form表单添加，添加一个边框阴影 */
    .form:focus-within {
      box-shadow: 0 0 0 5px rgba(0, 0, 0, 50%);
    }
    /* 去掉outline的周围的黑色原生边框 */
    .first-input:focus {
      outline: none;
    }
  </style>
</head>
<body>
  <div class="form">
    <input type="text" class="first-input">
  </div>
</body>
</html>